Distribution in the 4D parameter space of the GSP-Spec reference grid, that contains the 51 373 synthetic spectra adopted for the stellar parametrisation. The colour-code refers to the number of available spectra in each 2D projection. For the derivation of the chemical abundance of a given chemical element X with the GAUGUIN method, 21 spectra are computed for most combinations of the four atmospheric parameters by varying the individual abundance of X (12 different species were considered: N, Mg, Si, S, Ca, Ti, Cr, Fe, Ni, Zr, Ce, Nd).